Wikipedia

Classical Definition Drying is a mass transfer process consisting of the removal of water moisture or moisture from another solvent, by evaporation from a solid, semi-solid or liquid (hereafter product). To be considered "drying", the final product must be a solid, either as a continuous sheet (e.g., paper), as long pieces (eg. wood), particles (cereal grains, corn flakes) or in powder form (sand, salt, washing powder, milk powder). To achieve this, there must be a source of heat, and a sink of the vapor thus produced. In bioproducts (food, grains, vaccines), and pharmaceuticals, the solvent to be removed is almost invariably water In the most common case, a gas stream, e.g., air, applies the heat by convection and carries away the vapor as humidity. Other possibilities are vacuum drying, where heat is supplied by conduction or radiation (or microwaves) while the vapor thus produced is removed by the vacuum system. Another indirect technique is drum drying (used, for instance, for manufacturing potato flakes), where a heated surface is used to provide the energy and aspirators draw the vapor outside the room. Freeze drying or lyophilization is a drying method where the solvent is frozen prior to drying and is then sublimed, i.e., passed to the gas phase directly from the solid phase, below the melting point of the solvent. Freeze drying is often carried out under high vacuum to allow drying to proceed at a reasonable rate. This process avoids collapse of the solid structure, leading to a low density, highly porous product, able to regain the solvent quickly. In biological materials or foods, freeze drying is regarded as one of the best if not the best method to retain the initial properties. It was first used industrially to produce dehydrated vaccines, and to bring dehydrated blood for assistance of war casualties. Now freeze drying is increasingly used to preserve some foods, especially for backpackers going to remote areas. The method may keep protein quality intact. Vitamins and bioactive compounds are more likely to be retained as well. In turn, the mechanical extraction of the solvent, e.g., water, by centrifugation, is not considered "drying". Extended Definition (Particularly applicable to food drying) As the main application of drying, food drying is a process whereby water in a product (phase I) is transferred to the dehydrating medium (phase II) as a consequence of a difference in water chemical potential. The water flux goes always from the phase with the higher chemical potential to that of the lower chemical potential. If temperature differences at both sides of the interface are not considered, it can be said that the driving force for drying is a positive difference between water activities in product and dehydrating medium. The water activity of air is the percent relative humidity divided by 100. This definition encompasses methods as diverse as air drying, vacuum drying, and osmotic drying. In the latter, foods are dried in a hypertonic solution of sugar (mainly fruits) and/or salt (mostly vegetables). If the solvent to be removed is water, the terms drying and dehydration become synonymous. In fact dehydration is a latin word that means "water removal". There is very extensive technical literature on this subject, including several major textbooks and a dedicated scientific journal (Drying Technology ). Drying mechanism In some products having a high initial moisture content, an initial linear reduction of the average product moisture content as a function of time may be observed for a short time. If drying is continued, the slope of the curve, the drying rate, becomes less steep (falling rate period) and eventually tends to a horizontal, at very long times, to become in equilibrium with the dehydrating medium. In the falling rate period, water migration from the product interior to the surface is mostly by molecular diffusion, i,e. the water flux is proportional to the moisture content gradient. This means that water moves from zones with higher moisture content to zones with lower values, a phenomenon explained by the second law of thermodynamics. If water removal is considerable, products undergo shrinkage and deformation, except in a well designed freeze drying process.

Read more at Wikipedia...